The objectives of this research are to determine: 1) the possible role of suppressor T cells in the regulation of experimental autoimmune thyroiditis (EAT), 2) the basis for genetic difference between Strain 2 and Strain 13 guinea pigs in susceptibility to EAT by a) determining the cellular requirements for transfer of EAT, b) determining whether cells from low dose-immunized Strain 13 guinea pigs can transfer EAT and c) determining whether low doses of thyroglobulin might activate suppressor T cells in Strain 13 animals and 3) the mechanisms involved in the potentiation of EAT by antithyroglobulin antiserum by examining the site and target of action of the potentiating antiserum. Suppressor T cells will be induced by immunization with guinea pig thyroglobulin (GPTG) in incomplete Freund's adjuvant. T and B cells from these treated animals will be purified by nylon wool or anti-Fab column fractionation and will be transferred to normal syngeneic guinea pigs. Similar methods will be used to transfer EAT with B and/or T cells from GPTG-sensitized donors to normal recipients. The mechanism of action of potentiating antibody will be assessed by immunofluorescent studies, plaque forming cell (PFC) assays and rosette formation.